1. Field of the Invention
The present invention relates to optical scanning apparatuses and image forming apparatuses. More particularly, the present invention pertains to scanning apparatuses suitable for use in image forming apparatuses employing an electrophotography process, such as laser beam printers, digital photocopiers, and multifunction printers, and to such image forming apparatuses.
2. Description of the Related Art
In an optical scanning apparatus used in a laser beam printer (LBP) or the like, a light source unit emits a light beam after modulation thereof performed in accordance with an image signal, and a rotatable polygonal mirror, which serves as a deflecting unit, periodically deflects the light beam. An imaging optical system having an fθ characteristic focuses the deflected light beam to a spot (imaging spot) on the surface of a photosensitive recording medium (photosensitive drum), and scans the surface with the imaging spot. Thus, image recording is performed.
Such an optical scanning apparatus has a synchronous detection sensor (synchronous detection element), which serves as a photodetector. The synchronous detection sensor adjusts the timing at which image formation on the surface of the photosensitive drum starts (hereinafter, “image-forming timing”), before the imaging spot is scanned over the surface of the photosensitive drum.
The synchronous detection sensor receives a synchronous detection light beam, which is a portion of a light beam deflected by the deflecting unit. A synchronous detection signal is detected from an output signal of the synchronous detection sensor. The image-forming timing is adjusted according to the synchronous detection signal.
Optical scanning apparatuses these days have an optical deflector having two deflecting surfaces. Light beams incident on the deflecting surfaces are simultaneously deflected to be scanned over the surfaces of the photosensitive drums. Such optical scanning apparatuses have been developed to meet the demand for reduction in size of these apparatuses. Japanese Patent Laid-Open No. 2002-055292 discloses an example of such optical scanning apparatuses.
Such optical scanning apparatuses have reduced the number of optical deflectors used in such an optical scanning apparatus, which previously had to be the same number as the light beams, to half. Thus, the above-described structure contributed to a reduction in size of optical scanning apparatuses.
Japanese Patent Laid-Open Nos. 2003-222812 and 2005-017680 disclose optical scanning apparatuses in which a synchronous detection element and a light source unit are attached to the same substrate. This structure contributes to a further reduction in size of the above-described optical scanning apparatuses.
FIG. 14 is a schematic view of an important part of the optical scanning apparatus disclosed in Japanese Patent Laid-Open No. 2005-017680.
The optical scanning apparatus shown in FIG. 14 has an optical box 200, in which laser emitting devices 121 and 122, an optical deflector 140, and a synchronous detection element 171, and the like are provided.
In FIG. 14, a light beam emitted from the laser emitting device 121 is directed to the synchronous detection element 171 via a reflecting mirror 156, an optical deflector 140, a reflecting mirror 163, and a synchronous detection optical system 164. Similarly, a light beam emitted from the laser emitting device 122 is directed to the synchronous detection element 171 via a reflecting mirror 155, the optical deflector 140, again the reflecting mirror 155, and a synchronous detection optical system 161. The synchronous detection element 171 detects each of the light beams emitted from the laser emitting devices 121 and 122 to control the image-forming timing.
FIG. 14 also shows imaging optical systems 151 and 162 having fθ characteristics.
The optical scanning apparatus shown in FIG. 14 deflects light beams with two deflecting surfaces of the optical deflector 140 to reduce the size of the optical scanning apparatus. The synchronous detection element 171 and the laser emitting device 122 are attached to the same substrate to further reduce the size of the optical scanning apparatus.
Image forming apparatuses (printing apparatuses) employing an electrophotography process these days require higher image quality. In particular, in color photocopiers, occurrence of color misregistration is a serious problem in obtaining high quality images.
One of the causes of color misregistration is variation in the position at which the imaging spot forms an image (hereinafter, an “image-forming position”), among the surfaces to be scanned (hereinafter, “scan surfaces”) of the photosensitive drums for each color. Variation in image-forming position is caused by an error in the shape of components, an error in installation of the components into a printing apparatus, or an error in the shape of a housing to which the components are attached. Such an error in the shape or installation causes variation in image-forming timing, and hence, variation in image-forming position, among the scan surfaces for each color. To minimize the variation in image-forming timing among the scan surfaces for each color, the focal length of the synchronous detection optical system needs to be increased. Two reasons for this are described below.
First, there may be a difference in detection timing due to an error in installation, in the scanning direction, of a synchronous detection slit. FIGS. 15A and 15B are schematic views of a synchronous detection optical system, showing a light beam is deflected and scanned by a deflecting unit. FIG. 15A shows an ideal state in which a synchronous detection slit (hereinafter, a “BD slit”) 11 is installed with no error in installation. FIG. 15B shows a state in which the BD slit 11 is installed with an error in installation, i.e., installed at a position shifted from the correct position by an amount ΔY in the scanning direction. The scanning speed Vs over the BD slit 11 is expressed as follows:Vs=(fb/f)×Vd where fb is the focal length of a synchronous detection optical element 9 in the main scanning direction, f is the focal length of an imaging optical system 15 in the main scanning direction, and Vd is the scanning speed over a scan surface 8. By increasing fb, Vs is increased.
The difference in detection timing, ΔT, at which the light beam passes through the BD slit 11, between the case in which the BD slit 11 is installed with no error in installation (FIG. 15A) and the case in which the BD slit 11 is installed with an error in installation (FIG. 15B) is expressed as follows:ΔT=ΔY/Vs Thus, even if an error in installation of an amount ΔY occurs, the difference in detection timing, ΔT, from the ideal nominal value established in the design phase, can be minimized by increasing Vs. By increasing fb, Vs is increased. As a result, the absolute magnitude of the difference in detection timing, ΔT, is reduced, whereby the relative amount of color misregistration occurring when images of each color are superposed is reduced.
Second, there is a difference in detection timing due to an error in installation of the synchronous detection slit in a direction in which the light beam travels, occurring when a multibeam light source is used. FIG. 16A shows light beams emitted from two multibeam light sources reaching the BD slit 11. These two multibeams are emitted from two light sources separated by the largest distance, in the main scanning direction. FIG. 16B shows a state in which the BD slit 11 is installed with an error in installation, i.e., installed at a position shifted from the correct position by an amount ΔX in the direction in which the light beams travel.
Let us assume that the position of the light beam on the BD slit 11 is shifted by an amount ΔY because the position of the BD slit 11 is shifted by an amount ΔX. The difference in the detection timing, ΔT, at which the light beam passes through the BD slit 11, between the case in which the BD slit 11 is installed with no error in installation (FIG. 16A) and the case in which the BD slit 11 is installed with an error in installation (FIG. 16B) is expressed as follows:ΔT=ΔY/Vs Thus, even if an error in installation of an amount ΔY occurs, the difference in detection timing, ΔT, from the ideal nominal value established in the design phase can be minimized by increasing Vs. By increasing fb, Vs is increased. As a result, the absolute magnitude of the difference in detection timing, ΔT, is reduced, whereby the relative amount of color misregistration occurring when images of each color are superposed is reduced.
In addition, even if an error in installation of an amount ΔX occurs, the amount of error in installation, ΔY, can be minimized by increasing fb. Thus, increasing fb is effective for reducing the difference in detection timing, ΔT, from the standpoints of ΔY and Vs.
Because of these reasons, increasing fb is effective for reducing the relative amount of misregistration of image-forming position among the scan surfaces for each color.
However, the accuracy of synchronous detection has not been sufficiently considered in known optical scanning apparatuses.